The idea of feeding in at the output end of the pi is described in the book
by Pappenfus et al. By using a highpass filter, you can use small (25uH)
inductors: you just need a 5 or 10,000 pF capacitor rated at several amps of
RF and the full HV plus a bit. One point that is made in the book is that
you want some protection on the HV, as if the capacitors arc, they drag the
HV, and discharge the power supply cap into them. If they're vacuum
variables, they probably die - expensively. If they are air variables, they
may still die, but they may protect the tube from flashover - depending on
the relative lead lengths and the other 627 imponderables! But you can use a
Swiss file and clean up the plates. As Rich said, you need at least double
the working voltage on the capacitor(s), and that includes any padders.
There was a design in the RSGB Bulletin about 35 years ago which did this,
but put 1000pF capacitors in series with the variables to try and get the DC
off them. This needs a few megohms across the variable C for it to work.
73
Peter G3RZP
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